Optical devices and fiber laser devices

Abstract

The optical device comprises: a resonator optical fiber having an HR-FBG; a resonator optical fiber having an OC-FBG; and an amplification optical fiber, the amplification optical fiber having an end connected to the resonator optical fiber and an end connected to the resonator optical fiber, and the standardized frequency is above 5.13, the amplification optical fiber is wound in a manner that does not cross on one plane and the end is located inside, and has: an outer section that is wound in a circular shape; and an inner section that is wound in a manner that forms at least two arc portions, and the curvature radius of the arc portion is set in a manner that causes a mode higher in order than the LP11 mode to suffer loss.

Description

Technical Field

[0001] The invention relates to an optical device and an optical fiber laser apparatus.

[0002] This application claims priority based on Japanese Patent Application No. 2020-072971 filed on April 15, 2020, and uses the contents thereof herein. Background Art

[0003] In recent years, fiber laser devices have attracted attention in various fields such as processing, automobile, and medical fields. Compared with existing laser devices (such as carbon dioxide laser devices), fiber laser devices have excellent beam quality and focusing properties. In addition, fiber laser devices do not require spatial optical components, so there are no problems such as alignment and maintenance.

[0004] Patent document 1 discloses a fiber laser device, which sets a filter section for an amplifying optical fiber with rare earth ions added to the core, so that only a specific high-order mode is propagated in addition to the fundamental mode. In this fiber laser device, by propagating a specific high-order mode, rare earth ions distributed in an area other than the center of the core and near the center of the core (i.e., an area near the outer periphery of the core) can also contribute to the amplification of light. Therefore, the beam quality of the emitted light can be maintained and the amplification efficiency of the rare earth ions can be improved.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2018-190834 Summary of the invention

[0008] 1. Technical issues to be resolved

[0009] However, in high-output fiber laser devices, the TMI (Transverse Mode Instability, also known as Thermal Modal Instability) phenomenon may occur. The TMI phenomenon refers to the fact that if the power of the excitation light injected into the amplification optical fiber is increased, a periodic thermal grating caused by quantum loss will be generated, and inter-mode coupling from the fundamental mode to the higher-order mode will occur, which will suppress the output of the signal light (laser) or cause the beam quality of the signal light to decrease. The TMI phenomenon is not considered at all in Patent Document 1, and there is room for improvement in improving the conversion efficiency from excitation light to signal light.

[0010] The present invention is proposed in view of the above situation, and its purpose is to provide an optical device and a fiber laser device that can improve the conversion efficiency from excitation light to signal light.

[0011] (II) Technical solution

[0012] In order to solve the above-mentioned problems, an optical device of the first mode of the present invention comprises: a first optical fiber (13), which has a first reflection portion (13a); a second optical fiber (15), which has a second reflection portion (15a); and an amplification optical fiber (14), which has a first end (14a) connected to the first optical fiber and a second end (14b) connected to the second optical fiber, and the standardized frequency is greater than 5.13, and the amplification optical fiber is wound in a manner that does not cross on a plane and the first end is located on the inner side, and has: an outer section (SC1) that is wound in a circular shape; and an inner section (SC2) that is wound in a manner that forms at least two first arc portions (AR1), and the first arc portion (AR1) is set with a curvature radius in a manner that causes a mode higher in order than the LP11 mode to suffer loss.

[0013] In the optical device of the above-mentioned method, the amplification optical fiber is wound in a manner that does not cross on one plane and the first end is located inside. The amplification optical fiber has: an outer section, which is wound in a circular shape; and an inner section, which is wound in a manner that forms at least two first arc portions, and the curvature radius of the first arc portion is set in a manner that causes a mode higher in order than the LP11 mode to suffer loss. By removing the mode higher in order than the LP11 mode generated at the connection point between the first optical fiber and the first end of the amplification optical fiber in the first arc portion of the inner section, the formation of a thermal grating is suppressed and the TMI phenomenon is prevented. As a result, the conversion efficiency from the excitation light to the signal light can be improved. In addition, since the amplification optical fibers do not cross on one plane (by winding in a manner that the first end is located inside), the temperature of the amplification optical fibers is not easy to rise compared to the case where the amplification optical fibers cross, thereby improving the conversion efficiency.

[0014] In the optical device of the above aspect, the inner section may be wound so that the first arc portion and a second arc portion (AR2) having a larger curvature radius than the first arc portion are alternately presented.

[0015] Furthermore, in the optical device according to the above aspect, the inner section may be wound in an elliptical shape.

[0016] Furthermore, in the optical device of the above aspect, at least two of the first arc portions may be formed at positions at which the distance from the first end is shorter than the coherence length.

[0017] In the optical device of the above-described embodiment, the amplification optical fiber may be a few-mode optical fiber capable of propagating light in multiple modes, and the first arc portion may be bent in such a manner that light in a mode higher than a predetermined mode is removed from the core of the amplification optical fiber.

[0018] Furthermore, in the optical device according to the above aspect, the reflectivity of the first reflecting portion may be higher than the reflectivity of the second reflecting portion.

[0019] Furthermore, in the optical device of the above aspect, it may include: a first reinforcing portion (Q1) that accommodates the first end; and a second reinforcing portion (Q2) that accommodates the second end.

[0020] In addition, in the optical device of the above-mentioned method, it may be that the second end is arranged outside the outer interval, and at least two third arc portions (AR3) are formed between the outer interval and the second end, and the curvature radius of the third arc portion (AR3) is set in a manner that causes a mode higher than the LP11 mode to suffer losses.

[0021] The fiber laser device (1 to 3) of the second mode of the present invention comprises: an optical device as described in any one of the above items; a plurality of excitation light sources (11, 18); and a beam combiner (12, 19), which couples the excitation light emitted from the plurality of excitation light sources with the optical device.

[0022] Furthermore, in the fiber laser device of the above aspect, the beam combiner may couple the excitation light to at least one of the first optical fiber and the second optical fiber of the optical device.

[0023] (III) Beneficial effects

[0024] According to the above aspect of the present invention, there is an effect of being able to improve the conversion efficiency from excitation light to signal light. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a diagram showing the main structure of the fiber laser device according to the first embodiment of the present invention.

[0026] Figure 2 It is a top view of the optical device according to the first embodiment of the present invention.

[0027] Figure 3 This is an explanatory diagram showing an amplification optical fiber to which a permanent twist is applied in the first embodiment of the present invention.

[0028] Figure 4 It is a diagram showing the main structure of a fiber laser device according to a second embodiment of the present invention.

[0029] Figure 5It is a top view of an optical device according to a second embodiment of the present invention.

[0030] Figure 6 It is a diagram showing the main structure of a fiber laser device according to a third embodiment of the present invention.

[0031] Fig. 7A It is a top view of an optical device included in a fiber laser device of a comparative example.

[0032] Figure 7B It is a top view of an optical device included in a fiber laser device according to another comparative example.

[0033] Figure 8 This is a graph showing the relationship between the input current and the conversion efficiency of the fiber laser devices of the embodiment and the comparative example.

[0034] Fig. 9 It is a graph showing the spectrum of the excitation light source used in Examples and Comparative Examples. DETAILED DESCRIPTION

[0035] Hereinafter, the optical device and the fiber laser device according to the embodiments of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the following embodiments.

[0036] (First Embodiment)

[0037] (Fiber laser device)

[0038] Figure 1 FIG. 2 is a diagram showing the main structure of the optical fiber laser device according to the first embodiment of the present invention. Figure 1 As shown, the fiber laser device 1 of this embodiment includes a plurality of excitation light sources 11, a beam combiner 12, a resonator optical fiber 13 (first optical fiber), an amplification optical fiber 14, a resonator optical fiber 15 (second optical fiber), a transmission optical fiber 16, and an output end 17. Such a fiber laser device 1 is a so-called front excitation type fiber laser device.

[0039] Here, the resonator optical fiber 13, the amplification optical fiber 14, and the resonator optical fiber 15 constitute an optical device R. The optical device R is a resonator that generates laser light, i.e., signal light, using the excitation light output by the excitation light source 11. In addition, the optical device R may also include other components (such as a transmission optical fiber 16, an output terminal 17, etc.).

[0040] In the present specification, when viewed from the amplification optical fiber 14 , the excitation light source 11 side may be referred to as “front”, and the output end 17 side may be referred to as “rear”.

[0041] In addition, Figure 1 In the figure, the fusion splice portions of various optical fibers are indicated by × marks. The fusion splice portions are actually arranged at the reinforcing portions Q1 and Q2 (see Figure 2 ) and is protected inside. The reinforcement parts Q1 and Q2 include, for example: an optical fiber receiving body having a groove capable of receiving the optical fiber; and a resin that fixes the various optical fibers to the optical fiber receiving body when the fusion splice part is received in the groove of the optical fiber receiving body. Figure 1 In other figures, the fusion-spliced ​​portions of various optical fibers are also indicated by × marks.

[0042] The excitation light source 11 outputs excitation light (front excitation light). The number of excitation light sources 11 can be set to any number according to the power of the laser output from the output end 17 of the optical fiber laser device 1. As the excitation light source 11, for example, a laser diode can be used. The beam combiner 12 couples the excitation light output by each excitation light source 11 with the front end of the optical device R (the front end of the resonator optical fiber 13).

[0043] The front end of the resonator optical fiber 13 is fusion-connected to the combiner 12, and the rear end of the resonator optical fiber 13 is fusion-connected to the front end 14a (first end) of the amplification optical fiber 14. In addition, the fusion-connected portion between the resonator optical fiber 13 and the amplification optical fiber 14 is referred to as the "fusion-connected portion F1" below. An HR-FBG (High Reflectivity-Fiber Bragg Grating) 13a (first reflection portion) is formed in the core of the resonator optical fiber 13. The HR-FBG 13 is adjusted to reflect light of the wavelength of the signal light in the light emitted by the active element of the amplification optical fiber 14 in the excited state with a reflectivity of approximately 100%. The HR-FBG 13a is a structure in which the high refractive index portion is repeated at a certain period along its long side direction.

[0044] The amplification optical fiber 14 has a core to which one or more active elements are added, a first cladding covering the core, a second cladding covering the first cladding, and a coating covering the second cladding. That is, the amplification optical fiber 14 is a double-clad optical fiber. As the active element added to the core, for example, rare earth elements such as erbium (Er), ytterbium (Yb), or neodymium (Nd) are used. These active elements emit light in an excited state.

[0045] As the core and the first cladding, silica glass or the like can be used. As the second cladding, a resin such as a polymer can be used. As the coating, a resin material such as an acrylic resin or a silicone resin can be used. The amplification optical fiber 14 is a so-called few-mode optical fiber that can propagate multiple modes. The number of modes propagated by the few-mode optical fiber is, for example, 2 or more and 25 or less.

[0046] The standardized frequency of the amplifying optical fiber 14 is preferably 3.83 or more. Preferably, the standardized frequency of the amplifying optical fiber 14 is expected to be 5.13 or more. Compared with the amplifying optical fiber with a standardized frequency of 3.83 or more, the amplifying optical fiber with a standardized frequency of 5.13 or more can enlarge the core diameter and increase the execution cross-sectional area of ​​the optical fiber, thereby suppressing the nonlinear optical effect. Here, the standardized frequency is an indicator representing the specifications of the optical fiber, which specifies the number of modes propagating in the optical fiber. The larger the standardized frequency, the more the number of modes that can propagate in the optical fiber, and the smaller the standardized frequency, the fewer the number of modes that can propagate in the optical fiber.

[0047] When the normalized frequency is less than 5.13 (with 3 significant figures), a total of 4 modes can be propagated in the optical fiber, namely, the LP02 mode, LP21 mode, LP11 mode as high-order modes, and the LP01 mode as the fundamental mode. When the normalized frequency of the amplifying optical fiber 14 is 5.13 or more, in addition to the above 4 modes, higher-order modes can also be propagated in the amplifying optical fiber 14.

[0048] In addition, when the normalized frequency is less than 3.83 (with 3 significant figures), the modes that can propagate in the optical fiber are a total of two modes, namely, the LP11 mode and the LP01 mode. When the normalized frequency is less than 2.40 (with 3 significant figures), the mode that can propagate in the optical fiber is only the LP01 mode. In addition, for details of the normalized frequency, please refer to the aforementioned Patent Document 1, for example.

[0049] A mode filter MF1 is provided near the fusion splice F1 in the amplification optical fiber 14. The mode filter MF1 is provided to remove the high-order mode (high-order mode that may cause the TMI phenomenon) generated at the fusion splice F1 between the resonator optical fiber 13 and the amplification optical fiber 14, and to suppress the TMI phenomenon. The mode filter MF1 is realized, for example, by bending a portion of the amplification optical fiber 14, and is designed to be able to remove higher-order modes (such as LP02 mode, LP21 mode, etc.) than the LP11 mode. That is, the mode filter MF1 is designed to allow the LP01 mode and the LP11 mode to pass. In addition, the specific structure of the optical device R including the mode filter MF1 will be described later.

[0050] The front end of the resonator optical fiber 15 is fusion-connected to the rear end 14b (second end) of the amplification optical fiber 14, and the rear end of the resonator optical fiber 15 is fusion-connected to the front end of the transmission optical fiber 16. In addition, the fusion-connected portion between the resonator optical fiber 15 and the amplification optical fiber 14 is hereinafter referred to as the "fusion-connected portion F2". An OC-FBG (Output Coupler-Fiber Bragg Grating) 15a (second reflection portion) is formed in the core of the resonator optical fiber 15. The OC-FBG 15a has a structure substantially the same as that of the HR-FBG 13a, and is adjusted to reflect light at a lower reflectivity than the HR-FBG 13a. For example, the OC-FBG 15a is adjusted to have a reflectivity of 10 to 20% for light of the wavelength of the signal light.

[0051] In the amplification optical fiber 14, the signal light reflected by the HR-FBG 13a and the OC-FBG 15a is in the longitudinal direction of the amplification optical fiber 14. The signal light is amplified by this reciprocation and becomes laser light. In this way, in the optical device R, the light is amplified to generate signal light (laser light).

[0052] The transmission optical fiber 16 transmits the laser light generated in the optical device R. The transmission optical fiber 16 includes a core, a cladding surrounding the core, and a coating covering the cladding. For example, a single-mode optical fiber can be used as the transmission optical fiber 16. The transmission optical fiber 16 can be, for example, a multi-mode optical fiber or a few-mode optical fiber.

[0053] The output end 17 is connected to the rear end of the transmission optical fiber 16 and emits the laser light transmitted from the transmission optical fiber 16. The output end 17 includes a columnar body (light-transmitting columnar member) that transmits the laser light transmitted from the transmission optical fiber 16. This member is a so-called end cap.

[0054] (Optical Devices)

[0055] Figure 2 is a top view of an optical device according to a first embodiment of the present invention. Figure 2 In the Figure 1 The same structures as shown are marked with the same reference numerals. Figure 2 As shown, the optical device R includes: a resonator optical fiber 13, an amplification optical fiber 14, and a resonator optical fiber 15, and the optical device R is mounted on a heat sink P. The heat sink P is, for example, a flat plate-shaped member formed of a material with high thermal conductivity (such as aluminum, copper, etc.). The heat sink P is provided to cool the optical device R. In addition, the heat sink P may have a flow path inside which a refrigerant can flow.

[0056] The amplification optical fiber 14 is wound so as not to cross on one plane (on the heat sink P). Specifically, the amplification optical fiber 14 is wound in one direction (on one plane) so that the end 14a (first end) is located at the innermost circumference and the end 14b (second end) is located at the outermost circumference and does not cross on one plane (on the heat sink P). Figure 2 In this example, the amplification optical fiber 14 is wound rightward (clockwise) from the end 14b. That is, the amplification optical fiber 14 is wound so that the fusion splice F1 between the amplification optical fiber 14 and the resonator optical fiber 13 is located at the innermost circumference, and the fusion splice F2 between the amplification optical fiber 14 and the resonator optical fiber 15 is located at the outermost circumference.

[0057] The resonator optical fiber 13 connected to the amplification optical fiber 14 is extended to the outside of the amplification optical fiber 14 (at Figure 2 In the example, it extends to the left side of the paper. Figure 2 In the example, the HR-FBG 13a is arranged outside the amplification optical fiber 14, but the HR-FBG 13a may be arranged inside the amplification optical fiber 14. The resonator optical fiber 15 connected to the amplification optical fiber 14 extends to the right side of the paper.

[0058] Here, the resonator optical fibers 13 and 15 do not substantially contain rare earth elements and are less likely to generate heat than the amplification optical fiber 14. Figure 2 As shown, even if the resonator optical fiber 13 is arranged to intersect the amplification optical fiber 14 , the heat dissipation effect of the entire optical device R can be substantially maintained. Therefore, damage caused by heat generated in the amplification optical fiber 14 can be reduced.

[0059] In addition, a reinforcement part Q1 (first reinforcement part) for reinforcing the fusion connection part F1 may be provided, and a reinforcement part Q2 (second reinforcement part) for reinforcing the fusion connection part F2 may also be provided. In order to prevent the fusion connection parts F1, F2 and the vicinity of the fusion connection parts F1, F2 from being damaged, as shown in FIG. Figure 2 As illustrated, the reinforcement parts Q1 and Q2 may have a certain straight line portion.

[0060] In addition, the amplification optical fiber 14 has an outer section SC1 and an inner section SC2. The outer section SC1 is a section formed into a coil shape having a space inside by being wound in a circular shape (spiral shape) around the axis O. In the outer section SC1, the radius of curvature of the amplification optical fiber 14 gradually decreases from the end 14b to the end 14a, and is the smallest at the innermost circumference of the outer section SC1 (the boundary between the outer section SC1 and the inner section SC2).

[0061] The inner section SC2 is arranged in a space inside the outer section SC1 in a coil shape (vortex shape). The inner section SC2 is a section in which the amplification optical fiber 14 is wound in a manner to form at least two arc portions, and the radius of curvature of the arc portion is set in a manner that causes a mode higher in order than the LP11 mode to suffer loss. Here, the end 14a of the amplification optical fiber 14 defines the end point of the inner section SC2, and is located at the innermost circumference of the amplification optical fiber 14 as described above. Therefore, it can be said that the amplification optical fiber 14 is wound so that the end 14a is closer to the inside than the area surrounded by the inner section SC2 of the amplification optical fiber 14.

[0062] Figure 2 The illustrated inner section SC2 is wound in an elliptical shape for one and a half turns. Figure 2 The illustrated inner section SC2 is wound from the end 14a to the outer section SC1 to alternately present: an arc portion AR1 (first arc portion) having a curvature radius set in such a manner that a mode higher in order than the LP11 mode suffers loss; and an arc portion AR2 (second arc portion) having a larger curvature radius than the arc portion AR1.

[0063] The arc portion AR1 of the inner section SC2 is as follows Figure 1 The portion where the mode filter MF1 shown in the figure functions. That is, the arc portion AR1 of the inner section SC2 is provided to remove the high-order mode (high-order mode that may cause TMI phenomenon) generated at the fusion connection portion F1 between the resonator optical fiber 13 and the amplification optical fiber 14. The purpose of providing at least two such arc portions AR1 in the inner section SC2 is to fully remove the above-mentioned high-order mode. In addition, by winding in a manner in which the arc portions AR1 and AR2 are alternately presented, it is possible to form a mode filter MF1 in which the amplification optical fiber 14 does not contact the reinforcement portion Q1 and the high-order mode can be fully lost.

[0064] like Figure 2 As shown in the example, by winding the inner section SC2 into an ellipse, the total length of the arc portion AR1 (the total arc length of the arc portion AR1) can be made longer than one circumference of a circle having the radius of curvature of the arc portion AR1, and the reinforcing portion Q1 having a straight portion can be arranged on the inner side of the wound amplification optical fiber 14. By setting the total length of the arc portion AR1 to longer than one circumference of a circle having the radius of curvature of the arc portion AR1, the high-order mode can be sufficiently lost. Here, for example, when the length of the arc portion AR1 is only half the circumference immediately after the fusion splice F1, it is necessary to consider that if the amplification optical fiber 14 reaches a high temperature, the high-order mode may not be sufficiently lost. In addition, the radius of curvature of the arc portion AR2 only needs to be larger than the radius of curvature of the arc portion AR1, and may be larger or smaller than the minimum value of the radius of curvature of the outer section SC1. Alternatively, the arc portion AR2 may be straight.

[0065] Here, the minimum value r of the curvature radius of the arc portion AR1 is min The curvature radius r may be any value that does not degrade the quality of the light beam propagating in the amplification optical fiber 14 and satisfies the mechanical reliability of the amplification optical fiber 14. s The curvature radius r that does not reduce the quality of the beam propagating in the amplification optical fiber 14 is less than 2 times. b It can be defined as the radius of curvature that can remove light of a higher-order mode than a predetermined mode from the core of the amplification optical fiber 14. For example, when the predetermined mode is a higher-order mode light than the LP11 mode, the radius of curvature r b This is the radius of curvature at which mode light of a higher order than the LP11 mode can be removed from the core of the amplification optical fiber (incur loss).

[0066] The curvature radius r that satisfies the mechanical reliability of the amplification optical fiber 14 is s It can be defined as the radius of curvature at which the amplification optical fiber 14 breaks when bent. b and the radius of curvature r s The larger of the two is set as the radius of curvature r f , then it can satisfy r f <r min ≤2r s The minimum value r of the curvature radius of the arc AR1 is set by min The minimum value r of the radius of curvature of the arc portion AR1 is set so as to satisfy the above equation. min , thereby ensuring the desired beam quality and mechanical strength. In addition, the space occupied by the amplification optical fiber 14 can be further reduced. That is, it is possible to ensure the desired beam quality and mechanical strength, and provide a more compact optical component.

[0067] The mode filter MF1 (arc portion AR1) is preferably arranged near the location where the high-order mode is generated, that is, the fusion splice F1. The reason is as follows. That is, with regard to the optical fiber laser device 1 such as the present embodiment, in order to suppress the nonlinear optical effect, the longitudinal mode (oscillation wavelength) is usually also multi-mode. Therefore, in the fusion splice F1, mode coupling from the fundamental mode (LP01 mode) to the high-order mode is generated regardless of the wavelength. The high-order mode generated in the fusion splice F1 will be waveguided for a short time, that is, lose coherence due to the difference in propagation speed corresponding to the wavelength.

[0068] That is, at a distance of more than the coherence length (the duration of the beat caused by inter-mode interference) from the fusion splice F1, each fundamental mode and the higher-order mode lose coherence, so the thermal barrier becomes smaller, resulting in a decrease in coupling to the higher-order mode. Even if the higher-order mode is removed at a distance of more than the coherence length, since coupling to the higher-order mode occurs before reaching the place, it is difficult to obtain the effect of suppressing the reduction in efficiency. For this reason, the mode filter MF1 (arc portion AR1) is preferably arranged near the fusion splice F1.

[0069] According to this, the interval between the fusion splice F1 (end 14a) and the mode filter MF1 (arc portion AR1) only needs to be shorter than the coherence length for light of the wavelength of the signal light propagating in the amplification optical fiber 14. In addition, although the length of the mode filter MF1 (the total length of the arc portions AR1) is as long as possible, the mode filter MF1 (at least two arc portions AR1) may be arranged in a region between the fusion splice F1 (end 14a) and a position away from the fusion splice F1 (end 14a) by the coherence length.

[0070] In addition, when a reinforcement portion Q1 is provided to reinforce the fusion splice F1, the mode filter MF1 (arc portion AR1) is preferably provided immediately after the end portion behind the reinforcement portion Q1. In addition, the portion of the amplification optical fiber 14 close to the fusion splice F1 may be easily changed in optical characteristics due to heat. In such a case, it is preferable to make the interval between the fusion splice F1 (end portion 14a) and the mode filter MF1 (arc portion AR1) longer. However, the interval may be shorter than the above-mentioned coherence length.

[0071] Here, if the effective refractive index N of any two modes propagating in the amplification optical fiber 14 is eff The difference is set as ΔN eff , assuming that the propagation wavelength of the amplification optical fiber 14 is λ and the wavelength width is Δλ, the duration length of the beat generated by inter-mode interference, i.e., the coherence length Lc, is expressed by the following formula (1).

[0072] Lc=2 / ΔN eff ×λ 2 / Δλ…(1)

[0073] For example, in the core of the amplification optical fiber 14 with a relative refractive index difference of 0.3%, a core medium refractive index of 1.45, and a core diameter of 50 μm, the LP01 mode and the LP02 mode with a propagation wavelength λ of 1070 nm and a wavelength width Δλ of 1 nm are propagated in the amplification optical fiber 14. The effective refractive index difference ΔN between the LP01 mode and the LP02 mode at this time is eff= 0.00028. According to the above formula (1), the coherence length Lc, which is the duration of the beat caused by the inter-mode interference, is 8.18 m.

[0074] Here, a permanent twist may be applied to the amplification optical fiber 14 . Figure 3 FIG. 1 is an explanatory diagram showing an amplification optical fiber to which permanent twist is applied in the first embodiment of the present invention. Figure 3 In the example, the amplification optical fiber 14 is a double-clad optical fiber having a core 21, a first cladding 22 covering the outside of the core 21, and a second cladding 23 covering the outside of the first cladding 22. In a cross section perpendicular to the longitudinal direction of the amplification optical fiber 14, the cross section of the first cladding 22 is a regular heptagon (non-circular cladding).

[0075] Since the boundary between the first cladding 22 and the second cladding 23 is a regular heptagon that is not symmetrical in the second order, it is possible to suppress the so-called oblique light, which is the light that is waveguided in the cladding but does not reach the core and does not function as the excitation light. However, if the cross-sectional shape of the first cladding 22 is only set to a polygon, the oblique light still remains. In order to further reduce the oblique light and further improve the utilization efficiency of the excitation light, the amplification optical fiber 14 is wound so as to be twisted in the circumferential direction around the central axis of the amplification optical fiber 14.

[0076] Since the amplifying optical fiber 14 is twisted, the orientation of the regular heptagon of the first cladding 22 will differ depending on the position of the amplifying optical fiber 14. Figure 3 As shown, the incident angles θ1, θ2, and θ3 (reflection angles) of the oblique light Ls vary according to the position in the length direction of the amplification optical fiber 14. That is, θ1≠θ2≠θ3. Due to the changes in the incident angles and reflection angles, the oblique light Ls can be incident on the core 21 while traveling in the first cladding 22, and function as excitation light. As a result, the utilization efficiency of the excitation light can be improved, and the power of the output light can be increased.

[0077] If the twist amount of the amplifying optical fiber 14 is too large, the problem of reduced beam quality will arise. Therefore, the twist amount of the amplifying optical fiber 14 is preferably less than 3 rotations / m. In this case, the degradation of the beam quality can be suppressed, specifically, the increase in the M2 value can be suppressed. The M2 value is an indicator that quantitatively represents the deviation from the Gaussian beam, indicating that the beam can be converged to several times the diffraction limit. In addition, the twist amount can be constant over the entire length of the amplifying optical fiber 14. Alternatively, the twist amount of the amplifying optical fiber 14 can be set to increase as it approaches the HR-FBG13a.

[0078] Here, the occurrence of the TMI phenomenon is closely related to the generation of a heat grid. As parameters that affect the generation of a heat grid, for example, the following parameters can be cited.

[0079] αpump : Absorption rate of the excitation light in the amplification optical fiber 14;

[0080] P pump : The power of the excitation light input to the amplification optical fiber 14;

[0081] a: ratio of signal optical density to excitation optical density;

[0082] ω: beat modulation degree (difference or ratio between peaks and valleys in fluctuation of light intensity in the amplification optical fiber 14 caused by inter-mode interference).

[0083] For the TMI phenomenon, α pump , P pump The larger the ω is, the easier it is to see; the smaller the a is, the easier it is to see.

[0084] As described above, in this embodiment, the amplification optical fiber 14 having a standardized frequency of 5.13 or more is wound on one plane without crossing and the end 14a is located at the innermost circumference. The amplification optical fiber 14 has an outer section SC1 wound in a circular shape and an inner section SC2 wound inside the outer section SC1. At least two arc portions AR1 are formed in the inner section SC2, and the curvature radius of the arc portion AR1 is set so that a higher-order mode than the LP11 mode suffers loss, and the arc portion AR1 functions as a mode filter MF1.

[0085] A high-order mode (a high-order mode that may cause the TMI phenomenon) generated at the fusion-spliced ​​portion F1 between the resonator optical fiber 13 and the amplification optical fiber 14 is removed by a mode filter MF1 provided near the fusion-spliced ​​portion F1 in the amplification optical fiber 14. For example, a mode (e.g., LP02 mode, LP21 mode, etc.) higher than the LP11 mode propagating in the core of the amplification optical fiber 14 is removed by the mode filter MF1.

[0086] Thus, the beat modulation degree ω among the above parameters can be reduced, so the occurrence of the TMI phenomenon can be suppressed, and as a result, the conversion efficiency from the excitation light to the signal light can be improved. In addition, in the present embodiment, the reinforcing portion Q1 having a straight portion can be arranged on the inner side of the wound amplification optical fiber 14, so that the reliability (mechanical reliability) can be improved. In addition, by making the amplification optical fibers not cross on one plane (by winding in a manner such that the first end is located on the inner side), the temperature rise of the amplification optical fibers can be suppressed compared to the case where the amplification optical fibers cross, so the conversion efficiency can be improved.

[0087] (Second Embodiment)

[0088] (Fiber laser device)

[0089] Figure 4FIG. 2 is a diagram showing the main structure of a fiber laser device according to a second embodiment of the present invention. Figure 4 In the Figure 1 The same reference numerals are used to denote the same structures as those shown in the figure. Figure 1 The fiber laser device 1 shown is different in that the amplification optical fiber 14 is formed with a mode filter MF2 in addition to the mode filter MF1 .

[0090] The mode filter MF2 is provided near the fusion splice F2 in the amplification optical fiber 14. The mode filter MF2 removes the high-order mode generated at the fusion splice F2 between the amplification optical fiber 14 and the resonator optical fiber 15. The mode filter MF2 is realized by, for example, bending a part of the amplification optical fiber 14, similarly to the mode filter MF1, and is designed to remove higher-order modes (for example, LP02 mode, LP21 mode, etc.) than the LP11 mode. That is, the mode filter MF2 is also designed so that only the LP01 mode and the LP11 mode can pass.

[0091] (Optical Devices)

[0092] Figure 5 FIG. 1 is a top view of an optical device according to a second embodiment of the present invention. Figure 5 In the Figure 2 The same structures as shown are marked with the same reference numerals. Figure 5 The optical device R shown has at least two arc portions AR3 (third arc portions) formed between the outer section SC1 and the end portion 14b of the amplification optical fiber 14. The arc portions AR3 have a curvature radius set so that a higher-order mode than the LP11 mode suffers loss.

[0093] The arc portion AR3 is used as Figure 4 The arc portion AR3 is a portion where the mode filter MF2 shown in the figure functions. That is, the arc portion AR3 is provided to remove the high-order mode generated at the fusion-connected portion F2 between the amplification optical fiber 14 and the resonator optical fiber 15. The arc portion AR3 is provided at least two times to sufficiently remove the high-order mode. The arc portion AR3 is formed, for example, to have the same curvature radius as the arc portion AR1 formed in the inner section SC2.

[0094] exist Figure 5In the example shown, the optical device R is formed to present, in order from the outer section SC1 to the end 14b: an arc portion AR3, an arc portion AR4 having a larger curvature radius than the arc portion AR3, and an arc portion AR3. Here, the arc portion AR4 is a portion provided to connect the two arc portions AR3, and is provided on a plane without intersecting itself and without intersecting the two arc portions AR3. That is, the mode filter MF2 can be composed of an amplifying optical fiber 14 wound in a circular shape (vortex shape). The total length of the arc portion AR3 is, for example, more than one circumference of a circle having a curvature radius of the arc portion AR3.

[0095] The mode filter MF2 (arc portion AR3) is preferably arranged near the fusion splice F2 where the high-order mode is generated. In addition, the interval between the fusion splice F2 (end 14b) and the mode filter MF2 (arc portion AR3) may be shorter than the above-mentioned coherence length for the light of the wavelength of the signal light propagating in the amplification optical fiber 14. In addition, although the length of the mode filter MF2 (the total length of the arc portion AR3) is as long as possible, the mode filter MF2 (arc portion AR3) may be arranged in the area between the fusion splice F2 (end 14b) and the position away from the fusion splice F2 (end 14b) by the coherence length.

[0096] When a reinforcement portion Q2 is provided to reinforce the fusion splice F2, the mode filter MF2 (arc portion AR3) is preferably provided immediately before the front end of the reinforcement portion Q2. Also, as in the first embodiment, a permanent twist may be applied to the amplification optical fiber 14.

[0097] In addition, in the present embodiment, the amplification optical fiber 14 having a standardized frequency of 5.13 or more is also wound in a plane without crossing and with the end 14a located at the innermost circumference. The amplification optical fiber 14 has an outer section SC1 wound in a circular (spiral) shape, and an inner section SC2 wound inside the outer section SC1. At least two arc portions AR1 are formed in the inner section SC2, and the radius of curvature of the arc portion AR1 is set in such a way that a mode higher in order than the LP11 mode suffers loss. The arc portion AR1 functions as a mode filter MF1, and a mode higher in order than the LP11 mode (e.g., LP02 mode, LP21 mode, etc.) is removed by the mode filter MF1.

[0098] In addition, in the present embodiment, at least two arc portions AR3 are formed between the outer section SC1 and the end portion 14b of the amplification optical fiber 14, and the radius of curvature of the arc portion AR3 is set so that a mode higher in order than the LP11 mode suffers loss. The arc portion AR3 functions as a mode filter MF2, and a mode higher in order than the LP11 mode (e.g., LP02 mode, LP21 mode, etc.) is removed by the mode filter MF2.

[0099] As described above, the aforementioned beat modulation degree ω can be reduced, so the occurrence of the TMI phenomenon can be suppressed, and as a result, the conversion efficiency from the excitation light to the signal light can be improved. In addition, in the present embodiment, the reinforcing portion Q1 having a straight portion can be arranged on the inner side of the wound amplification optical fiber 14, so that the reliability (mechanical reliability) can be improved. In addition, since the amplification optical fibers do not cross on one plane (by winding in a manner such that the first end is located on the inner side), the temperature of the amplification optical fibers is less likely to rise compared to the case where the amplification optical fibers cross, so the conversion efficiency can be improved.

[0100] (Third Embodiment)

[0101] (Fiber laser device)

[0102] Figure 6 FIG. 2 is a diagram showing the main structure of a fiber laser device according to a third embodiment of the present invention. Figure 6 In the Figure 4 The same reference numerals are used to denote the same structures as those shown in the figure. Figure 4 The fiber laser device 2 shown is different in that it includes a plurality of excitation light sources 18 and a beam combiner 19. Such a fiber laser device 3 is a so-called bidirectional excitation type fiber laser device.

[0103] The excitation light source 18 outputs excitation light (rear excitation light). The number of excitation light sources 18 can be set to any number according to the power of the laser output from the output end 17 of the fiber laser device 3. As the excitation light source 18, a laser diode can be used, for example, as with the excitation light source 11. The beam combiner 19 couples the excitation light output by each excitation light source 18 to the rear end of the optical device R (the rear end of the resonator optical fiber 15).

[0104] The optical device R of the fiber laser device 3 of this embodiment has the same structure as the optical device R of the fiber laser device 2 of the second embodiment. However, the optical device R of the fiber laser device 3 of this embodiment is different from the optical device R of the fiber laser device 2 of the second embodiment in that not only the front excitation light but also the rear excitation light is supplied.

[0105] In addition, in the present embodiment, the amplification optical fiber 14 having a standardized frequency of 5.13 or more is also wound in a plane without crossing and with the end 14a located at the innermost circumference. The amplification optical fiber 14 has an outer section SC1 wound in a circular (spiral) shape, and an inner section SC2 wound inside the outer section SC1. At least two arc portions AR1 are formed in the inner section SC2, and the radius of curvature of the arc portion AR1 is set in such a way that a mode higher in order than the LP11 mode suffers loss. The arc portion AR1 functions as a mode filter MF1, and a mode higher in order than the LP11 mode (e.g., LP02 mode, LP21 mode, etc.) is removed by the mode filter MF1.

[0106] In addition, in the present embodiment, at least two arc portions AR3 are formed between the outer section SC1 and the end portion 14b of the amplification optical fiber 14, and the radius of curvature of the arc portion AR3 is set so that a mode higher in order than the LP11 mode suffers loss. The arc portion AR3 functions as a mode filter MF2, and a mode higher in order than the LP11 mode (e.g., LP02 mode, LP21 mode, etc.) is removed by the mode filter MF2.

[0107] As described above, the aforementioned beat modulation degree ω can be reduced, so the occurrence of the TMI phenomenon can be suppressed, and as a result, the conversion efficiency from the excitation light to the signal light can be improved. In addition, in the present embodiment, the reinforcing portion Q1 having a straight portion can be arranged on the inner side of the wound amplification optical fiber 14, so that the reliability (mechanical reliability) can be improved. In addition, since the amplification optical fibers do not cross on one plane (by winding in a manner such that the first end is located on the inner side), the temperature of the amplification optical fibers is less likely to rise compared to the case where the amplification optical fibers cross, so the conversion efficiency can be improved.

[0108] Example

[0109] The inventor of this case prepared the fiber laser device 1 of the first embodiment as an example. That is, as an example, a fiber laser device 1 having an optical device R is prepared, and the optical device R is composed of an amplification optical fiber 14 formed with a mode filter MF1 and resonator optical fibers 13 and 15. The amplification optical fiber 14 has a standardized frequency of 5.13 or more and Yb is added to the core. The amplification optical fiber 14 is as follows: Figure 2 It is wound as shown, and has a coil-shaped (spiral-shaped) outer section SC1 and an inner section SC2 where an arc portion AR1 functioning as a mode filter MF1 is formed.

[0110] The curvature radius of the arc portion AR1 is set so that a mode higher in order than the LP11 mode (e.g., LP02 mode, LP21 mode, etc.) suffers loss (is removed). The reflectivity of the HR-FBG 13a in the resonator optical fiber 13 is set to 99%, and the reflectivity of the OC-FBG 15a in the resonator optical fiber 15 is set to 10%. Fig. 9 Graph showing the spectrum of the excitation light source used in the embodiment and the comparative example. Fig. 9 As shown, the wavelength at which the intensity of the excitation light emitted from the excitation light source 11 reaches a peak is 975 nm.

[0111] As a comparative example, a Fig. 7A , Figure 7B The fiber laser device of the optical devices R1 and R2 of the structure shown in the figure has the same other structure as the fiber laser device 1 of the above embodiment. Fig. 7A , Figure 7B : is a top view of an optical device included in a fiber laser device of a comparative example. Fig. 7A The optical device R1 shown is provided in the optical fiber laser device of Comparative Example 1. Figure 7B The optical device R2 shown is provided in the fiber laser device of Comparative Example 2.

[0112] like Fig. 7A As shown, the optical device R1 of the fiber laser device of Comparative Example 1 does not have an arc portion AR1 whose curvature radius is set so that a mode higher than the LP11 mode suffers loss. In addition, the fusion splice portion F1 of the resonator optical fiber 13 and the amplification optical fiber 14 and the reinforcement portion Q1 are arranged outside the wound amplification optical fiber 14.

[0113] like Figure 7B As shown, in the optical device R2 of the fiber laser apparatus of Comparative Example 2, an arc portion AR1 having a curvature radius set so that a higher-order mode than the LP11 mode suffers loss is formed at only one location. The length of the arc portion AR1 is half the length of a circle having the curvature radius of the arc portion AR1.

[0114] Figure 8 This is a graph showing the relationship between input current and conversion efficiency in the fiber laser devices of the embodiment and the comparative example. Figure 8 The horizontal axis of the graph is the current [A] input to the excitation light source 11 , and the vertical axis is the conversion efficiency [%] from the excitation light to the laser light. The larger the value on the horizontal axis, the greater the power of the excitation light input to the amplification optical fiber 14 .

[0115] like Figure 8It is shown that: for the fiber laser device of comparative example 1, when the input current exceeds 36A, the linearity between the input current and the conversion efficiency is destroyed, and the conversion efficiency drops sharply. It is also confirmed that the conversion efficiency drops sharply until the input current reaches 40A. In addition, for the fiber laser device of comparative example 2, even if the input current exceeds 36A, the linearity between the input current and the conversion efficiency is basically maintained, and the reduction in conversion efficiency can be suppressed. However, it is confirmed that: when the input current exceeds 40A, the conversion efficiency drops sharply. In contrast, in the fiber laser device of the embodiment, there is no sharp drop in conversion efficiency even if the input current exceeds 40A.

[0116] In this embodiment and the comparative example, an excitation light having a wavelength of 975 nm reaching a peak intensity is used. This wavelength (975 nm) coincides with the absorption peak wavelength of the Yb-doped amplification optical fiber 14, so it is considered that thermal gradation caused by a beat occurs in the comparative example 1, and the conversion efficiency is reduced due to the aforementioned TMI phenomenon.

[0117] It is considered that in Comparative Example 2, Figure 7B The arc portion AR1 shown removes the redundant high-order mode to a certain extent, thereby suppressing the formation of the thermal grating caused by the beat. However, in Comparative Example 2, the arc portion AR1 is not long enough, and the removal of the high-order mode is insufficient. Therefore, it is considered that when the input current exceeds 40A, thermal grading caused by the beat will occur, resulting in a decrease in conversion efficiency due to the above-mentioned TMI phenomenon.

[0118] In contrast, in the embodiment, it is considered that the mode filter MF1 (plural arc portions AR1) can be used to remove unnecessary high-order modes, thereby suppressing the formation of a thermal barrier caused by a beat. It is therefore considered that even when the input current exceeds 40A, the formation of a thermal barrier caused by a beat can be suppressed, and the conversion efficiency will not be sharply reduced due to the aforementioned TMI phenomenon. In this way, it can be confirmed that the mode filter MF1 (plural arc portions AR1) is used to fully remove unnecessary high-order modes.

[0119] Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments and can be freely modified within the scope of the present invention.

[0120] For example, in Figure 2 , Figure 5 The HR-FBG 13 a and the OC-FBG 15 a may be interchanged in the optical device R shown. That is, the OC-FBG 15 a may be formed in the resonator optical fiber 13 , and the HR-FBG 13 a may be formed in the resonator optical fiber 15 .

[0121] Furthermore, the fiber laser devices 1 to 3 of the above-described embodiments have one output terminal 17, but an optical fiber or the like may be further connected before the output terminal 17. In addition, a beam combiner may be connected before the output terminal 17 to combine laser beams from a plurality of laser devices.

[0122] In addition, the fiber laser devices of the first and second embodiments are so-called front-excitation fiber laser devices, and the fiber laser device of the third embodiment is so-called bidirectional excitation fiber laser device. However, the fiber laser device may be a fiber laser device 3 (see FIG. 1 ) in which the third embodiment is omitted. Figure 6 ) is a so-called rear-excitation type fiber laser device that has an excitation light source 11 and a beam combiner 12.

[0123] Furthermore, components in the above-described embodiments may be appropriately replaced with known components without departing from the spirit of the present invention, and the above-described embodiments and modifications may be appropriately combined.

[0124] Description of Reference Numerals

[0125] 1-3…fiber laser device; 11…excitation light source; 12…beam combiner; 13…optical fiber for resonator (first optical fiber); 13a…HR-FBG (first reflection part); 14…optical fiber for amplification; 14a, 14b…ends; 15…optical fiber for resonator (second optical fiber); 15a…OC-FBG (second reflection part); 18…excitation light source; 19…beam combiner; AR1-AR3…arc portion; Q1, Q2…reinforcement portion; R…optical device; SC1…outer section; SC2…inner section.

Claims

1. An optical device comprising: A first optical fiber having a first reflecting portion; a second optical fiber having a second reflecting portion; and an amplification optical fiber having a first end connected to the first optical fiber and a second end connected to the second optical fiber, and having a standardized frequency of 5.13 or more, The amplifying optical fiber is wound on a plane and does not cross on the plane, and the first end is located inside. The amplification optical fiber has an outer section wound in a circular shape and an inner section wound to form at least two first arc portions, the first arc portions having a curvature radius set so that a mode higher than the LP11 mode suffers loss.

2. The optical device according to claim 1, characterized in that: The inner section is wound such that the first arc portions and second arc portions having a larger curvature radius than the first arc portions are alternately presented.

3. The optical device according to claim 1 or 2, characterized in that: The inner section is wound into an elliptical shape.

4. The optical device according to any one of claims 1 to 3, characterized in that: At least two of the first arc portions are formed at positions at which the distance from the first end is shorter than a coherence length, wherein the coherence length refers to a duration length of a beat generated by inter-mode interference.

5. The optical device according to any one of claims 1 to 4, characterized in that: The amplification optical fiber is a few-mode optical fiber capable of propagating multiple modes of light. The first arc portion is bent so as to remove light in a mode higher in order than a predetermined mode from the core of the amplification optical fiber.

6. The optical device according to any one of claims 1 to 5, characterized in that: The reflectivity of the first reflecting portion is higher than the reflectivity of the second reflecting portion.

7. The optical device according to any one of claims 1 to 6, characterized in that: have: a first reinforcing portion receiving the first end; and The second reinforcing portion receives the second end.

8. The optical device according to any one of claims 1 to 7, characterized in that: The second end is arranged outside the outer section, At least two third arc portions are formed between the outer section and the second end, and the third arc portions have a curvature radius set so that a mode higher in order than the LP11 mode receives a loss.

9. A fiber laser device comprising: The optical device according to any one of claims 1 to 8; multiple excitation light sources; and A beam combiner is used to couple the excitation light emitted from the plurality of excitation light sources with the optical device.

10. The optical fiber laser device according to claim 9, characterized in that: The beam combiner couples the excitation light to at least one of the first optical fiber and the second optical fiber of the optical device.

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